The present invention relates generally to large commercial utility boilers or steam generators and, in particular, to a new and useful natural circulation steam generator which uses a plurality of large diameter vertical pressure vessels at the top of downcomers of the steam generator, instead of a conventional single large steam drum.
A conventional natural circulation boiler or steam generator system, generally designated 10, is schematically illustrated in FIG. 1. The system 10 comprises a steam drum 12, downcomers (DC""s) 14 provided with downcomer bottles (DCB""s) 15 at a lower end thereof, supply tubes 16, furnace wall tubes 18, riser tubes 20 and steam/water separators 22 inside the steam drum 12. Typically, heated feedwater 24 enters the drum 12 via a feedwater distribution system whose task it is to thoroughly mix the feedwater 24 with the saturated water in the steam drum 12 which has been separated from the steam-water mixture supplied to the separators 22 in the steam drum 12 via the riser tubes 20.
The resulting water mixture (usually subcooled, i.e., the temperature of the water is below the saturation temperature corresponding to the operating pressure within the steam drum 12) enters and flows down through the downcomers 14 and is distributed, via a number of supply tubes 16, to inlet headers 26 of the furnace circuits, e.g. the wall tubes 18.
Circulation of the water in the furnace circuits or wall tubes 18 (also shown in the FIGS. as front wall (FW), side walls (SW""s), and rear wall (RW), is established through the difference in fluid density between the subcooled water in the downcomers 14 and the steam/water mixture in the heated furnace circuits 18. The fluid velocity in the wall tubes comprising the furnace circuits must be sufficient to cool the furnace wall tubes 18, which are typically exposed to combustion gases B whose temperature may reach 3500xc2x0 F. in the burner zone 28 of the furnace.
As soon as the heated fluid reaches saturation conditions, steam begins to form and the fluid within the wall tubes 18 becomes a two-phase mixture. The fluid velocity must be sufficient to maintain nucleate boiling (bubble-type boiling) within the tubes 18, as this is the regime which generates the highest possible heat conductance, i.e. the best cooling between the fluid within the tubes 18 and the inside wall of the tubes 18 on the heated (furnace) side. Insufficient fluid velocity in combination with high heat flux and an excessive percentage of steam in the steam-water mixture leads to steam blanketing on the inside of the tubes 18. This is equivalent to an insulating-type steam film along the heated, inside tube wall, which causes rapid tube failure. The danger of film boiling increases with increasing boiler pressure. The fluid temperature in the boiling (two-phase) regime is strictly dependent on the local internal pressure and is nearly constant from the point where boiling starts to the point where the saturated water leaves the separators.
The steam-water mixture eventually reaches outlet headers 30 of the furnace circuits 18. From here, the steam-water mixture is conveyed to the steam drum 12 and distributed along a baffle space therein and from there to a plurality of steam/water separators 22 located inside the steam drum 12.
The steam/water separators 22 separate the saturated water from the saturated steam, usually through centrifugal force generated through either tangential entry of the two-phase fluid into cyclones or through stationary propeller-type devices. The centrifugal action literally xe2x80x9csqueezesxe2x80x9d the steam out of the steam-water mixture.
The saturated steam leaves the top of the steam drum 12 through saturated connecting tubes 32 which supply the steam to the superheater(s) 34 of the boiler or steam generator system 10, where the steam is further heated to the desired final temperature before being sent to a turbine or a process. The saturated water, as stated earlier, leaves the bottom of the steam/water separators 22 and mixes with the continuously supplied feedwater.
The crucial element in a conventional steam generator or boiler circulation system 10 is the steam drum 12. In high-pressure boilers with natural or pump-assisted circulation, such steam drums 12 may be over 100 feet long, with a 6 foot inside diameter, and shell thicknesses over 7 inches. Thus, the steam drums 12 are very large and extremely heavy and must be lifted in place as soon as the boiler and its structural steel and columns are erected, prior to erecting all other boiler pressure parts. Accordingly the steam drum 12 is on the critical path of the overall schedule for such boiler and power plant projects. For a more detailed description of conventional steam generators and steam drums as generally described above, the reader is referred to the 40th edition of Steam/its generation and use, 40th Edition, Copyright(copyright) 1992, The Babcock and Wilcox Company.
Elimination of large steam drums in favor of reduced size separating vessels, according to the present invention, is the logical consequence of a steady reduction in the so-called Circulation Ratio. The Circulation Ratio (CR) is defined as (total water flow to the furnace circuits/steam flow to the superheater). For many years, the minimum CR for natural (or pump-assisted) circulation high pressure ( greater than 2500 psig drum operating pressure) boilers was 4.0. However, the invention and successful introduction of multi-lead ribbed furnace tubes made it possible to reduce the CR, as ribbed tubes can safely operate at much lower water flow rates than internally smooth tubes exposed to furnace heat. Therefore, the drumless boiler concept according to the present invention becomes economical at CR""s below 3.0.
At the low end of the CR spectrum would be the types of steam generators known as subcritical once-through boilers which have a CR of 1.0 andxe2x80x94typically no separating equipment, except perhaps that used for removal of residual moisture. The present inventors believe, therefore, that it is only logical that with decreasing CR, a natural (or pump-assisted) circulation boiler design should more and more resemble a once-through subcritical boiler. As the following description of the drumless natural circulation boiler concept will demonstrate, this philosophy, and its attendant benefits, is realized by the present invention. As experience with the new type of drumless natural circulation boiler design becomes available, the present inventors believe that the future trend will be towards ever-decreasing CR""s, since lower CR""s require fewer and smaller, and therefore less expensive, connections (supply tubes, riser tubes, downcomers, etc.) and steam/water separators in the circulation system of such boilers.
An object of the present invention is to provide a drumless natural circulation boiler system. A crucial difference between such a system and a conventional natural circulation system with steam drum is that the single large steam drum is eliminated and the tops of the downcomers are modified into large vertical steam/water separators in the form of large diameter, vertically extending vessels. Phase separation is achieved through a suitable number of tangential nozzles which lead the steam-water mixture from the riser tubes into the separators where the saturated steam is separated from the steam-water mixture through centrifugal action along the separator""s cylindrical inside periphery. The nozzles must be suitably inclined against the horizontal plane to avoid interference between the multiple fluid jets. The tangential velocity is a function of the total flow to each separator, the boiler pressure, the number and size of the nozzles, the allowable pressure drop across the separators, and the inside diameter of the separators, and must be sufficient to effect separation, like in other types of separators. Preferably, the upper portion of the vertical steam/water separators is provided with an internal arrangement of vertical scrubber elements arranged around the inside perimeter of the vertical steam/water separators and through which the steam is conveyed to remove a significant portion of any water remaining in the steam.
Accordingly, one aspect of the present invention is drawn to a drumless natural circulation boiler system. The system comprises a furnace enclosure having wall tubes, and upper and lower headers connected to respective upper and lower ends of the wall tubes. At least one vertical steam/water separator is provided, and riser means are connected between the upper headers and the separator for returning a steam/water mixture to the separator, the riser means being connected to the separator for swirling the steam/water mixture in the separator for separating steam from water in the separator. Saturated steam connection means are connected to the separator for conveying saturated steam therefrom. A downcomer is connected to the separator for conveying water from the separator, and supply means are connected between the downcomer and the lower headers for conveying water thereto.
Another aspect of the present invention is drawn to the vertical steam/water separator itself. The vertical steam/water separator comprises a vertically extending cylindrical vessel having a top and a bottom portion. Means are provided for introducing a steam/water mixture to the vessel for swirling the steam/water mixture in the separator for separating steam from water in the separator. Vertically oriented scrubber means remove water from steam, and are located in the top portion of the vessel and arranged around an inside circumference of the separator. Saturated steam connection means convey saturated steam from the vessel, feedwater supply means convey feedwater to the vessel, and means are provided for conveying the feedwater and water separated from the steam from the vessel.
Yet another aspect of the present invention is drawn to a steam/water separator for a boiler which receives feedwater and a steam/water mixture, separates the steam from the water, conveys the separated steam from the separator, and mixes the feedwater with the separated water and conveys both from the separator. The separator comprises a vertically extending cylindrical vessel having a top and a bottom portion and defines a plurality of zones therein, each zone having a particular function. The zones include a secondary steam/water separation zone having scrubber means for removing a final portion of water from the steam. An entrainment separation zone is located below the scrubber means and above a boiler steam/water entry zone, the latter providing the steam/water mixture into the separator via a plurality of inclined tangential nozzles. A primary steam/water separation zone, located below the boiler steam/water entry zone, is where water spirals downwardly to the bottom of the separator. A vertical separator water level operation zone is located below the primary steam/water separation zone. This zone will be substantially filled with water having a fluctuating water level during boiler operation. A feedwater injection zone, located below the vertical separator water level zone, defines where the feedwater is introduced into the separator for mixing with the separated water. Finally, a lower vortex elimination zone, located below the feedwater injection zone, performs the function of reducing rotation of the feedwater and water as it is conveyed from the separator.
Advantages of the drumless natural circulation boiler design include the fact that the separators/downcomers will be straight, can be placed optimally around a furnace, and can be erected at a later stage, rather than immediately after erection of the boiler support frame. The cost for material, fabrication, shipping, and erection for separators is considerably less than for drums of equal capacity.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.